A food production wiki for public health professionals

Yogurt

Key Facts

Yogurt is made from milk cultured with live bacteria.

Yogurt is consumed in a variety of ways, including Greek yogurt, drinkable yogurt, and frozen yogurt.

The use of pasteurized milk is a key barrier to foodborne pathogen transmission in yogurt products.Raw milk can contain pathogens such as Escherichia coli, Salmonella spp., and Campylobacter jejuni.The acidity of yogurt is another barrier to foodborne illness. There is evidence of E. coli 0157:H7 exhibiting acid-tolerant properties, but this pathogen is readily destroyed via pasteurization.

Yogurt products have previously been associated with fungal disease.

Introduction

Yogurt is made from milk cultured with live bacteria. The bacteria produce lactic acid, which coagulates the milk proteins, making yogurt thick and slightly sour in flavor. Yogurt can be consumed directly, but is also used as a lower-calorie substitute in cooking and to make dips and dressings.

The bacterial cultures required for making yogurt are Streptococcus thermophilus and Lactobacillus bulgaricus. About 80% of all yogurt manufactured in the U.S. contains an additional culture, Lactobacillus acidophilus, and many commercial yogurt products also contain Bifidobacterium bifidum or Lactobacillus casei because of their potential health benefits.

New York leads the nation in yogurt production, accounting for 15.7% of total U.S. yogurt production, followed by California.

In 2008, per capita consumption of yogurt in the U.S. was 11.8 pounds.

Types of Yogurt

The U.S. Food and Drug Administration (FDA) regulates and inspects commercial yogurt products. The FDA also set the following guidelines:

Regular yogurt must be no less than 3.25% fat and contain no less than 8.25% milk solids.

Low fat yogurt must be between 0.5 to 2% fat and contain no less than 8.25% milk solids.

Non-fat yogurt must be less than 0.5% fat and contain no less than 8.25% milk solids.

Other terms used to describe yogurt are based on differences in processing:

Half heavy cream, half whole milk yogurt.

Set (equivalent to sundae or fruit-on-the-bottom): This is the firmest type of yogurt; during processing the yogurt is set in a container and is left undisturbed.Fruit can be added to the bottom of the container so that when turned upside down it resembles a sundae.

Swiss (equivalent to stirred, custard, or blended): Most commercial yogurts are Swiss-style. After the yogurt is set, it is stirred and dispensed into secondary containers, making it less firm than set yogurt. Fruit can also be stirred in to produce flavor varieties.

Liquid or drinkable: Liquid yogurt has milk, fruit, and/or fruit syrups added for increased fluidity and flavor. The pH of yogurt is raised when milk is added, so the shelf life of drinkable yogurt is generally only 4-10 days.

Frozen yogurt: To make frozen yogurt, regular yogurt is mixed with a pasteurized ice cream mix of milk, cream, and sugar. Other ingredients, such as stabilizers, fruit, and flavors are blended together and then the mixture is frozen. In some cases, frozen yogurt contains live bacteria; the bacteria become dormant when cooled, but regain activity post-ingestion due to the warm body temperature.

Greek yogurt: Greek yogurt contains about twice the protein and half as much carbohydrates and sodium as regular yogurt. During processing, the yogurt is strained three times, as opposed to two times with regular yogurt, which yields the higher protein content and creamier texture. There are also low-fat and fat-free options for Greek yogurt.

Lite or light: Yogurt that has 50% less fat or one-third fewer calories than regular yogurt is considered light.

Foodborne Outbreaks

In 2013, commercial yogurt products were determined by FDA to be contaminated with the fungi, Mucor circinelloides, and symptoms, including vomiting, nausea and diarrhea, were reported by more than 300 consumers. The risk associated with fungal pathogens is not well understood, but M. circinelloides may cause spoilage in yogurt, and it poses a particular risk to the immunocompromised.

Production

Ingredients

Milk is the main ingredient used when making yogurt. It can be cream, whole, low-fat, or skim. Whole milk is used to make full-fat or regular yogurt, low-fat milk is used to make low-fat yogurt, and skim milk is used to make non-fat yogurt. In general, the higher the fat content of the milk, the smoother and creamier the yogurt will taste.

Live cultures are the second key ingredient in yogurt. There are two required cultures, Streptococcus thermophilus and Lactobacillus bulgaricus, and several optional ones.

Cream may be added to adjust the fat content of milk. Non-fat, dry milk powder is used to adjust the solids content of the yogurt above the 8.25% minimum for better body and texture. Stabilizers function to improve body and texture as well as increase firmness. They also keep fruit uniformly mixed in the yogurt and prevent separation of whey. Examples of stabilizers include alginates, gelatin, gums, pectins, and starch.

Sugar, honey, or artificial sweeteners may be used to reduce the naturally sour flavor of yogurt. Fruit, fruit syrups, and pie filling are also optional and can either be mixed in with the yogurt or added to the top or bottom of the yogurt.

Processing

Pasteurization: In order to prevent deactivation of the barerial cultures needed in yogurt production, the milk is pasteurized at 185°F for 30 minutes or 203°F for 10 minutes prior to adding the cultures. . The high heat also denatures the whey proteins, which allows the yogurt to form a more stable gel. Lastly, pasteurization effectively kills disease-causing bacteria.

Adjusting Milk Composition and Blending Ingredients: At this point, stabilizers are added to the mixture, as are sweeteners, if a less tart yogurt is desired. The non-fat, dry milk powder is also added before heating to prevent coagulation of the milk proteins.

Homogenization: Not all yogurts are homogenized. If this step is taken, the ingredients are mixed well to ensure a more stable consistency.

Heating: The milk is then heated to 200°F for 10-20 minutes depending on the desired thickness of the yogurt. Holding it longer will result in a thicker yogurt.

Cooling and Inoculation: The mixture is then cooled rapidly to 112-115°F. At this point the warm mixture is inoculated with the live bacterial culture.

Incubation: The mixture is incubated for 4-7 hours at 105-115°F. The bacteria used in making yogurt are thermophilic and this is their optimal temperature range; they are killed above 130°F and do not grow well below 98°F. Yogurt will become firm when a pH of 4.6 is reached. Incubating the mixture any longer will result in an increased acidity and more sour taste.

Cooling: When the desired pH is reached, the yogurt is cooled to around 45°F to end the fermentation process.

Addition of Fruit and Flavors: For set style yogurt, the fruit is added to the bottom of the cup and the inoculated yogurt is placed on top before fermentation. For Swiss style yogurt, the fruit is mixed with the yogurt following fermentation and cooling. The yogurt is then packaged at which point it should be refrigerated at 40°F or lower.

Food Safety

The acidity of yogurt acts as a barrier to bacteria growth, as does the high temperature achieved during the yogurt-making process. However, milk must be pasteurized beforehand to sufficiently kill disease-causing pathogens, such as E. coli 0157:H7, which may be acid-tolerant.

It is essential that all equipment and work spaces used in the yogurt-making process remain clean and sanitized to prevent the addition of unwanted bacteria to the yogurt.

The FDA requires that yogurt be made with live cultures, but some yogurts are heat-treated so that the final product contains no active cultures. The label should specify what microorganisms are present and in what amount in terms of colony-forming units (CFUs) as well as the known health benefits associated with the particular strains used. The label should also disclose the expiration or use-by date, serving size suggestion, company name, and proper storage of the product. Regardless of the use-by date, yogurt with visible signs of microbial growth or off-odors should be discarded immediately.

The shelf life of yogurt is 10-21 days. For liquid yogurt, the shelf life is 4-10 days and for yogurt cheese the shelf life is 7-14 days when refrigerated at 40°F. Yogurt can also be frozen for several months, but this may alter its texture.

Consumption

In 2008, Americans consumed an average of 11.8 pounds of yogurt per person. Although yogurt consumption in the United States has continued to rise, the per capita consumption is dwarfed by Sweden’s annual 62.8 pounds per person. Research by the NPD Group report that Americans between the ages of 18-34 are driving the increase in yogurt consumption in comparison to the older generations. In recent years, the health benefits of yogurt (particularly the recognition of probiotics) has encouraged a higher consumption of yogurt and incorporation of yogurt into many more food products. Yogurt has been introduced into a wider variety of products including fast food establishment menus, toothpastes, beauty products, and even pet foods. In the 2006-2007 Foodnet Population Survey Atlas of Exposures, 43.3% of the survey cohort reported eating fresh or flavored store bought yogurt within the past 7 days.

Information on keeping yogurt fresh and safe to eat can be found at FoodKeeper App.

Nutrition

The health benefits associated with yogurt consumption are plentiful and there is ongoing research suggesting yogurt may contribute more to overall health than is currently known. All types of this fermented dairy product are a nutrient-rich source of calcium, potassium, and protein, and certain types of yogurt have additional benefits, e.g. when fortified with vitamin D and/or probiotics.

Probiotics are live microorganisms that develop a symbiotic relationship with the host when administered in the proper amount. Lactic acid bacteria that survive in the gut are often used as probiotics. This includes Lactobacillus rhamnosus, L. casei, L. acidophilus, and Bifidobacterium lactis, among others. The bacteria acquire nutrients and energy from the food people eat, and in return, help maintain a healthy gut microbiota. Research suggests this may promote immune function, improve mental health, and protect against cognitive impairment, and may lower the risk of some chronic diseases, such as cancer, high blood pressure, obesity, and diabetes. Different strains of bacteria confer different benefits, which may be specified on the product label.

Gastrointestinal Health: Consuming probiotics is associated with a reduced severity and duration of diarrhea in children with acute infectious diarrhea, in those taking antibiotics, and in those with lactose intolerances. There is also evidence that probiotic consumption may be helpful in managing the symptoms of irritable bowel syndrome (IBS) and reducing relapses of ulcerative colitis. Some probiotics inhibit the growth of Helicobacter pylori, which is associated with ulcers and cancer of the stomach. The role of probiotics in preventing traveler’s diarrhea is being investigated.

Immune Health: Researchers have demonstrated that consuming probiotics can decrease the incidence of upper respiratory infections in adults and reduce cold and flu-like symptoms in children (resulting in higher attendance in preschool and daycare). There is also evidence that consuming yogurt may lower the incidence of yeast infections.

Emerging Research: Changes in the gut microbiota may occur with obesity and type II diabetes. There is potential for probiotics to play a role in the prevention of obesity and diabetes, but more research is needed. There is also a new field of research devoted to studying the link between the consumption of certain probiotics and mental health. A recent study showed that probiotics may reduce anxiety and stress, but further research is needed to confirm these findings.